For example, a hot-rolled steel sheet used in cars, industrial machines and the like is generally manufactured through a rough-rolling process and a finish-rolling process. FIG. 18 is a view schematically illustrating a method for manufacturing a hot-rolled steel sheet of the related art. In the process for manufacturing a hot-rolled steel sheet, first, a slab S obtained by continuously casting molten steel having an adjusted predetermined composition is rolled using a roughing mill 101, and then, furthermore, hot-rolled using a finishing mill 103 constituted by a plurality of rolling stands 102a to 102d, thereby forming a hot-rolled steel sheet H having a predetermined thickness. In addition, the hot-rolled steel sheet H is cooled using cooling water supplied from a cooling apparatus 111, and then coiled into a coil shape using a coiling apparatus 112.
The cooling apparatus 111 is generally a facility for carrying out so-called laminar cooling on the hot-rolled steel sheet H transported from the finishing mill 103. The cooling apparatus 111 sprays the cooling water on the top surface of the hot-rolled steel sheet H moving on a run-out table from the top in the vertical direction in a water jet form through a cooling nozzle, and, simultaneously, sprays the cooling water on the bottom surface of the hot-rolled steel sheet H through a pipe laminar in a water jet form, thereby cooling the hot-rolled steel sheet H.
In addition, for example, Patent Document 1 discloses a technique of the related art which reduces the difference in surface temperature between the top and bottom surfaces of a thick steel sheet, thereby preventing the shape of the steel sheet from becoming defective. According to the technique disclosed in Patent Document 1, the water volume ratio of cooling water supplied to the top surface and the bottom surface of the steel sheet is adjusted based on the difference in surface temperature obtained by simultaneously measuring the surface temperatures of the top surface and the bottom surface of the steel sheet using a thermometer when the steel sheet is cooled using a cooling apparatus.
In addition, for example, Patent Document 2 discloses a technique that measures the steepness at the tip of a steel sheet using a steepness meter installed on the exit side of a mill, and prevents the steel sheet from being perforated by adjusting the flow rate of cooling water to be different in the width direction based on the measured steepness.
Furthermore, for example, Patent Document 3 discloses a technique that aims to solve distribution of a wave-shaped sheet thickness in the sheet width direction of a hot-rolled steel sheet and to make uniform the sheet thickness in the sheet width direction, and controls the difference between the maximum heat transmissibility and the minimum heat transmissibility in the sheet width direction of the hot-rolled steel sheet to be in a range of predetermined values.